The GNOME desktop is able to use a sequence of images for its background wallpaper, changing them automatically over time, but does not include an interface for specifying which images should constitute such a slideshow. Create Background Slideshow (CreBS) is a small Python/GTK application that provides such an interface, saving users from having to manually edit XML files.

If you use Ubuntu, you can install Create Background Slideshow from the CreBS PPA:
ppa:crebs/ppa